1. Field of the Invention
The present invention relates to processes for forming diffusion coatings. More particularly, this invention relates to a process and material capable of locally producing a diffusion coating on limited surface regions of a substrate.
2. Description of the Related Art
The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature capabilities have been achieved through the development of iron, nickel and cobalt-base superalloys and through the use of oxidation-resistant environmental coatings. Aluminum-containing coatings, particularly diffusion aluminide coatings, have found widespread use as environmental coatings on the external and internal surfaces of gas turbine engine components. Aluminide coatings are generally formed by a diffusion process such as pack cementation or vapor phase aluminizing (VPA) techniques, or by diffusing aluminum deposited by chemical vapor deposition (CVD) or slurry coating. Aluminide coatings contain MAl intermetallic (where M is the base material of the substrate, typically Ni, Co, or Fe), as well as other intermetallic phases formed by metals present in the substrate prior to aluminizing. Platinum aluminide (PtAl) diffusion coatings further contain platinum aluminide intermetallics and platinum in solution in the MAl phase as a result of plating platinum on the substrate prior to the aluminiding step. During high temperature exposure in air, these aluminide coatings form a protective aluminum oxide (alumina) scale that inhibits further oxidation of the coating and the underlying substrate.
Slurries used to form aluminide coatings contain an aluminum powder in an inorganic binder, and are directly applied to the surface to be aluminized. Aluminizing occurs as a result of heating the component in a non-oxidizing atmosphere or vacuum to a temperature that is maintained for a duration sufficient to melt the aluminum powder and diffuse the molten aluminum into the surface. As described in U.S. Pat. No. 6,444,054, slurry coatings may contain a carrier (activator), such as an alkali metal halide, which vaporizes and reacts with the aluminum powder to form a volatile aluminum halide, which then re-acts at the component surface to form the aluminide coating. The amount of slurry applied must be very carefully controlled because the thickness of the resulting aluminide coating is proportional to the amount of slurry applied to the surface. The difficulty of consistently producing diffusion aluminide coatings of uniform thickness has discouraged the use of slurry processes on components that require a very uniform diffusion coating and/or have complicated geometries, such as turbine blades.
In contrast to slurry processes, pack cementation and VPA processes are widely used to coat broad surface regions of airfoils and other gas turbine engine components because of their ability to form coatings of uniform thickness. Both of these processes generally entail reacting the surface of a component with an aluminum halide gas formed by reacting an activator (e.g., an ammonium or alkali metal halide) with an aluminum-containing source (donor) material. In pack cementation processes, the aluminum halide gas is produced by heating a powder mixture comprising the source material, activator, and an inert filler such as calcined alumina. The ingredients of the powder mixture are combined and then packed and pressed around the component to be treated, after which the component and powder mixture are heated to a temperature sufficient to vaporize the activator. The activator reacts with the source material to form the volatile aluminum halide, which then reacts at the component surface to form an aluminide coating. In contrast to pack processes, VPA processes are carried out with the source material (e.g., an aluminum alloy) placed out of contact with the surface to be aluminized.
There are occasions when only a localized region of a component requires coating. For example, if the tip of an airfoil has undergone repair (e.g., following return from service), only the repaired tip surface requires recoating. Another example is when one or more surface regions of a new-make airfoil (e.g., prior to installation and operation in an engine) remain uncoated following a line-of-sight coating process, such as physical vapor deposition (PVD). The above-noted processes for depositing diffusion coatings have limitations that make them less than ideal for producing localized diffusion coatings. For example, in order to coat local surface regions of a component using conventional vapor phase and pack cementation processes, extensive masking is required to prevent coating deposition on those surfaces that do not require coating. While the slurry process is capable of producing localized coatings without masking, the difficulty of controlling the thickness of the coating using slurries is a significant drawback, particularly if the coating is to be formed on surface areas with complex shapes.
Approaches have been proposed for overcoming the above shortcomings, including the use of pack cementation tapes. However, such tapes often have very low green strength, with the result that the tapes tend to delaminate during processing to yield coatings of variable quality. U.S. Pat. No. 6,110,262 to Kircher et al. proposes a localized cementation process that uses organic binders and solvents to contain the cementation powders against the part to be coated. However, the use of extraneous binding agents can lead to inconsistency in the coating process because the cohesion required of the binding agents to maintain the strength of the mixture may also create a barrier to the coating vapors. Other potential drawbacks include carbide formation or the introduction of other impurities into the coating during decomposition of an organic binder, and environmental issues if the organic binder contains a hazardous solvent, such as acetone, toluene, etc.
In view of the above, there is an ongoing need for processes capable of depositing a diffusion coating of uniform thickness on localized surface regions of a component.